1. Field of the Invention
This invention relates to a magnetic brush developing device, and more particularly to a magnetic brush developing device for use in an electrophotographic reproducing machine.
2. Discussion of the Prior Art
Magnetic brush developing devices are disclosed in U.S. Pat. Nos. 2,786,439 and 2,786,440 issued to C. J. Young in 1957. A magnetic brush developing device is very effective for reproducing continuous tone images, and because of this, the device is increasingly used in electrophotographic reproducing machines or the like.
Generally in magnetic brush development, a developer consisting of an iron powder carrier and toner is formed into brush-like burs by means of a developing roller consisting of a stationary magnet and a sleeve rotating around the stationary magnet so that the brush-like burs may slidingly pass over the surface of an electrostatic latent image bearing member thereby developing the electrostatic latent image into a visual image. Transportation of the developer is effected by the sleeve rotating about the magnet and hence the transportability of the developer is largely dependent upon the magnetic field produced by the magnet used.
The magnet used in the prior art is of a rectangular cross-section as shown in FIG. 1, a sleeve 2 being rotatably mounted around the rectangular magnet. A transporting magnet 3 delivers the developer to the succeeding magnet 1.
Because the rectangular cross-sectional shape magnet 1 has magnetic poles in the right and reverse sides thereof, flux density A in the vertical direction with respect to the developing magnetic pole (as shown by phantom line A) is high, while flux density B in the direction tangential to the magnetic pole (as shown by phantom line B) is low. Since flux density B in the tangential direction contributes to transportion of the developer, the transportability of the magnetic pole in the conventional device has been extremely low, and hence the permissible range of flow rate of developer has been extremely limited. For this reason, if an increase in the amount of developer to be transported is needed for increasing the developing speed, the magnetic pole in the conventional device will fail to transport the required amount of developer properly, with the resulting accumulation of developer in the supply side of developing roller, the accumulated developer being eventually spilt off from the roller to the outside of the developing device.